This library provides a way to declare IORefs, TVars, Chans, etc.
at the top level of a module.

Declarations are safe. User code does not invoke unsafePerformIO
or compiler pragmas.

Variables are ordinary module-level bindings. Their scope can be
controlled through the usual module import/export mechanism. Unrelated
code cannot interfere with private variables by guessing some global
identifier.

References are statically typed, and polymorphic references are statically
forbidden.

The implementation is simple, and closely follows existing common practice.

This library does not prevent the software design problems caused by global
state. Rather, it provides some implementation safety for those
circumstances where global state really is necessary.

Fundamentally this library just provides Template Haskell macros for the
usual unsafePerformIO trick, so it is subject to the same caveats. In
particular, linking a module and then loading the same module dynamically
through the GHC API may produce two copies of the same top-level variable.
See the documentation of unsafePerformIO for additional compiler-specific
requirements.